Natual air flow??? What kind of air flow do you think you have coming
from the soffits to the roof line? Wind tunner perhaps? Not.

Soffit vents are generally (I am saying generally from the ones I have
seen) are a few feet apart. They are suppose to help with heat
escaping
in the summer and keeping the roof cold in the winter so water doesnt
prematurely melt and create an ice backup. (ice dam)

That being said, putting a 6" hole in the soffit isnt going to disrupt
anything.
The forced air from the fan will blow the exhaust right out the vent.
More
or less a few feet away. The 'Natural Flow" of the soffit doesnt even
come close to making a measureable vacume effect that would suck in the
exhaust from the vent in the soffit.

One thing that can also bit people in the butt is that when venting
moist
warm air vertically, you increase the chance of condensation flowing
back
in from the vent. Lots of insulation would be advisable. (not sure if
your in
a warm climate or cold)

Horizon. runs you can pitch so that the flow wont come back.

Just a thought...

Tom

P.S.... I personally HATE putting holes in the room. More holes, more
chance of
water coming in. Over the years, the moisture from the showers and
rust
from the vent cause some crappy looking roofs too (stains)